Method for producing acrylate adhesive materials

ABSTRACT

A process for preparing pressure sensitive adhesives based on acrylate hotmelt, in which a monomer mixture including at least the following monomers 
 
(a) 70 to 100% by weight of compounds from the group of (meth)acrylic acid and the derivatives thereof corresponding to the following general formula  
                 
is free-radically polymerized in solution wherein I. the polymerization is initiated using at least one dissociating photoinitiator and by irradiation with ultraviolet light, the photoinitiator being added to the monomer mixture before the beginning of the polymerization and/or to the reaction mixture in the course of the polymerization, II. the polyacrylate is freed from the solvent, III. the polyacrylate is processed further in the melt.

The invention relates to a process for preparing acrylic hotmelts byfree-radical addition polymerization and also to the use of such anacrylic hotmelt.

Within the field of pressure sensitive adhesives (PSAs), as a result ofongoing technological developments in the coating process, there existsa continuing demand for innovative developments. In industry, hotmeltprocesses with solvent-free coating technology are of growing importancefor the preparation of PSAs, since the environmental impositions arebecoming ever greater and the prices of solvents continue to rise.Consequently solvents are to be eliminated as far as possible from themanufacturing operation for PSA tapes. As a result of the associatedintroduction of the hotmelt technology, the requirements imposed on theadhesives are becoming evermore stringent. Acrylic PSAs in particularare being investigated very intensively with a view to theirimprovement. For high-grade industrial applications there is apreference for polyacrylates, on account of their transparency andstability to weathering. Besides these advantages, however, theseacrylic PSAs must also meet exacting requirements in the areas of shearstrength and of bond strength.

Acrylic PSAs can be prepared with great individuality. They can beemployed very variably, since a large number of different comonomers areavailable for polymerization and since the adhesive properties of thepolyacrylate can be varied through the choice of the (co)monomercomposition. Comonomers used include as principal components commonlyalkyl esters of acrylic and methacrylic acid, in smaller fractionsusually (additionally) acrylic acid, methacrylic acid, acrylamides,maleic anhydride, hydroxy acrylates or itaconic acid. Thesepolyacrylates are prepared using free-radical addition polymerization insolution or in emulsion. Both techniques are subject, among otherthings, to the problems depicted below, but are very cost-effective andhave therefore long been carried out in production.

For processing as a hotmelt PSA it is necessary to remove the solvent.For emulsion polymers this is a very energy-intensive operation, sincewater has a low vapor pressure and is therefore very difficult toevaporate. Moreover, emulsifiers which remain in the polymer aredisruptive, raising the sensitivity of the resultant PSA to water.

In the case of solution polymers it is desirable to lower the solventfraction. Here, however, the conventional methods are limited, since thepolymerization is normally initiated thermally. During thepolymerization it is necessary to remove the developing heat of reactionby way of evaporative cooling. Since the initiators generally used forthe polymerization are azo initiators having decomposition half-lives ofapproximately 1 hour at T>60° C., moreover, fractions of initiatorremain in the polymer and are then present even after the concentrationoperation as well. These residual initiators may give rise to a varietyof problems. On the one hand it is possible, in the course of meltcoating at very high temperatures, for a large fraction of the residualinitiators to decompose. The nitrogen gas which is liberated producesbubbles within the PSA, which are undesirable for the quality of theproduct. On the other hand, polyfunctional acrylates are added in orderto ensure that the acrylic PSA is crosslinkable by electron beams. Theseacrylates are particularly essential for resin-blended acrylic PSAs. Inthe course of coating not only is there generation of bubbles by theresidual initiators but also the free radicals formed react with thepolyfunctional acrylates and so bring about an additional, uncontrolledcrosslinking of the acrylic PSA. This gelling, as it is known, has anadverse influence on coating. The partly gelled adhesive can no longerbe applied as a coating, and, moreover, relatively solid areas withincreased gelling may form in the adhesive, and adversely affect theadhesive properties of the PSA tape.

One solution is the UV polymerization of acrylates. U.S. Pat. No.3,840,448 polymerized acrylates and crosslinked them by the irradiationof UV light. The polymers prepared were not, however, used for PSAs.

U.S. Pat. No. 4,181,752 prepares UV prepolymers which followingapplication as a coating to the backing are polymerized to fullconversion and crosslinked. U.S. Pat. No. 4,968,558 and U.S. Pat. No.5,183,833 likewise prepared PSAs from acrylates and vinyl compounds byway of the prepolymerization technique. Prepolymerization was practisedtherein using UV-B lamps in conjunction with Irgacure 651™ (benzildimethyl ketal, Ciba Geigy). The prepolymerization technique is alreadyvery widespread but is also hampered by problems. A high monomer contentsyrup has to be applied to the backing material; only on the backingmaterial is polymerization carried out to high conversion. Because ofthis operation, only very slow web speeds can be achieved; moreover, theresidual monomer fraction in the adhesive is relatively high.Furthermore, the incorporation of resins causes problems, sincecrosslinking is generally carried out using UV light and many resins ofindustrial importance absorb UV light and so act as regulators.

It is an object of the invention, therefore, to offer a process forpreparing acrylic pressure sensitive adhesives which does not have theabovementioned disadvantages of the prior art. In particular it ought tobe possible to achieve a high conversion as early as duringpolymerization in solution without subsequent gelling processesoccurring in the resulting acrylate composition. The adhesive propertiesof the acrylate composition ought not to fall below a level which isnecessary for its technical application.

This object is achieved, surprisingly and unforeseeably for the skilledworker, by a process as set out in the main claim. The subclaims relateto advantageous developments of the invention and also to a use for thepressure sensitive adhesives prepared.

The invention accordingly provides a process for preparing pressuresensitive adhesives based on acrylate hotmelt, in which a monomermixture including at least the following monomers

-   (a) 70 to 100% by weight of compounds from the group of the    (meth)acrylic esters corresponding to the following general formula-    with R₁═H or CH₃ and R₂=an alkyl chain having 2 to 20 carbon atoms    and also including-   (b) 0 to 30% by weight of olefinically unsaturated monomers    containing functional groups and also including-   (c) if desired, further components    is free-radically polymerized in solution to give a polyacrylate,    and in which-   I. the polymerization is initiated using at least one dissociating    photoinitiator and by irradiation with ultraviolet light, the    photoinitiator being added to the monomer mixture before the    beginning of the polymerization and/or to the reaction mixture in    the course of the polymerization,-   II. the polyacrylate is freed from the solvent.-   III. the polyacrylate is processed further in the melt.

The composition of the corresponding comonomers is preferably chosen sothat the resultant PSAs possess pressure sensitive adhesive propertiesin accordance with D. Satas [Handbook of Pressure Sensitive AdhesiveTechnology, 1989, Verlag VAN NOSTRAND REINHOLD, New York]. The staticglass transition temperature of the PSA is therefore below 25° C.

For the inventive process for preparing the PSAs the comonomers used(component (b)) include olefinically unsaturated compounds whichpreferably contain functional groups, with a fraction of 0-30 percent byweight. Examples of such olefinically unsaturated compounds are(meth)acrylic acid and the methyl esters thereof, methacrylic acidderivatives such as (meth)acrylamides, N-substituted (meth)acrylamides,dimethylacrylic acid, trichloroacrylic acid, hydroxyalkyl(meth)acrylate, amino-containing (meth)acrylates, hydroxy-containing(meth)acrylates, with particular preference 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, and/or4-hydroxybutyl(meth)acrylate, acrylonitrile, and also vinyl compoundssuch as vinyl esters, vinyl ethers, vinyl halides, vinylidene halides,and nitriles of ethylenically unsaturated hydrocarbons, vinyl compoundswith aromatic rings and heterocycles in α position, particularlyvinylacetic acid and vinyl acetate, N-vinylformamide, vinylpyridine,ethyl vinyl ether, vinyl chloride, vinylidene chloride, and also maleicanhydride, styrene, styrene compounds, β-acryloyloxypropionic acid,fumaric acid, crotonic acid, aconitic acid and/or itaconic acid; theabove list is only exemplary and not conclusive.

As photoinitiators it is possible to use all Norrish type Iphotoinitiators (also referred to hereinbelow for short as type Iphotoinitiators). The fraction of the photoinitiators, based on themonomers used, is advantageously between 0.1 and 2, preferably between0.25 and 1 percent by weight. With preference it is possible, forexample, to use Irgacure 651™ or Irgacure 819™ (Ciba Geigy).Photoinitiator mixtures as well are very suitable for initiation in theinventive sense. Great preference is given to using photoinitiators withlongwave absorption, since these possess a great depth of penetrationand therefore penetrate the monomer/polymer mixture more readily.

For linear polymerization it is preferred to carry out initiation usinga Norrish type I photoinitiator. Norrish type II photoinitiators (typeII photoinitiators) cause a greater level of grafting reactions (for thepreparation of branched polyacrylates) and are therefore metered inpreferably in the course of the UV polymerization. Nevertheless, UVpolymerizations can also be initiated using type II photoinitiators.

Norrish type I photoinitiators are those compounds which dissociate inaccordance with a Norrish type I reaction when irradiated with light.Such a reaction is, conventionally, the photofragmentation of a carbonylcompound in the course of which the bond to a carbon atom located a tothe carbonyl group is free-radically cleaved (α cleavage), so forming anacyl radical and an alkyl radical.

In the inventive sense, the Norrish photoinitiators also include thosein which instead of the carbonyl group another functional group ispresent and cleavage affects the bond between this group and an a carbonatom.

Norrish type II photoinitiators dissociate on irradiation with light inaccordance with a Norrish type II reaction involving hydrogenabstraction, which is an intramolecular reaction.

In the case of aliphatic ketones it is possible in this instance for ahydrogen to be eliminated from the γ position to one corresponding tothe above-depicted functional group.

Inventive examples of Norrish photoinitiators of both types arebenzophenone, acetophenone, benzil, benzoin, hydroxyalkylphenone, phenylcyclohexyl ketone, anthraquinone, thioxanthone, triazine or fluorenonederivatives, without wishing to impose any unnecessary restriction bygiving this list. The type I initiators include in particular aromaticcarbonyl compounds, such as benzoin derivatives, benzil ketals, andacetophenone derivatives. Type II photoinitiators are in particulararomatic ketones, such as benzophenone, benzil or thioxanthones, forexample.

For further details see, for example, Römpp Lexikon Chemie-Version 2.0,Stuttgart/New York: Georg Thieme Verlag 1999.

The polymerization for preparing the acrylic PSA can be conducted inpolymerization reactors, which are generally provided with a stirrer,two or more feed vessels, a reflux condenser, heating and cooling andare equipped for operation under N₂ atmosphere and superatmosphericpressure. Moreover, a UV irradiation means should be integrated in thereactor.

The UV light is advantageously irradiated into the reactor from above.For mixing it is preferred to use a propeller stirrer which ensuresvertical mixing. Effective distribution of the radicals formed in thereaction mixture promotes the polymerization, whereas if mixing is poorthe polymerization results are likely to be unsatisfactory, owing to thelimited depth of penetration of the UV light into the reaction mixture.

The polymerization proceeds favorably in accordance with a free-radicalmechanism. The monomer mixture is irradiated with UV light preferablyunder an inert gas atmosphere, such as nitrogen, helium or argon, forexample.

The free-radical polymerization can be conducted in the presence of oneor more organic solvents and/or in the presence of water or in bulk,with preference being given to polymerization in the presence of atleast one organic solvent; in that case, it is advantageous to use aslittle solvent as possible. The polymerization time—depending onconversion and temperature—is between 1 and 24 h.

In the case of solution polymerization it is preferred as solvents touse esters of saturated carboxylic acids (such as ethyl acetate),aliphatic hydrocarbons (such as n-hexane or n-heptane), ketones (such asacetone or methyl ethyl ketone), special boiling point spirit ormixtures of these solvents. For the polymerization it is also possibleto use thiols, nitroxides, TEMPO derivatives(TEMPO=2,2,6,6-tetramethyl-1-piperidinyloxy pyrrolidinyloxyl)) orthioesters of any of a wide variety of kinds as further regulators forlowering molecular weight and reducing polydispersity. Also effective aspolymerization regulators, as they are known, are alcohols and ethers.

In one very favorable version of the inventive process the irradiationwith ultraviolet light is carried out not continuously but instead inthe form of one or more irradiation cycles. By means of the irradiationcycles of the UV source it is possible to control the polymerizationwith regard to the products formed. To control the polymerization it maybe appropriate not to irradiate continuously. In that case the sequenceof irradiation times and dark times can be chosen advantageously. Bymeans of corresponding irradiation cycles it is possible to control theconversion and hence also the amount of heat produced. The molecularweight distribution as well can be influenced by the irradiation cycle.Particularly advantageous is the irradiation of the monomers to bepolymerized with a defined pulse. This method results in the formationonly of a very few radicals which are able to react. By means of the UVpulse, accordingly, it is possible to control the number of radicalsproduced and hence also to adjust the monomer/radical ratio. With aconstant ratio of monomer concentration to radical concentration it ispossible using this technique to prepare polyacrylate PSAs with a narrowdistribution.

On the other hand it is also possible very rapidly to generate a highconcentration of radicals, which produce a high conversion. UV-initiatedpolymerizations therefore proceed much more quickly and economicallythan conventional polymerizations using azobisisobutyronitrile (AIBN) asinitiator.

A further advantage is the simple process technique. Thermally initiatedfree radical polymerizations with AIBN, for example, generate heat inthe course of the polymerization, as a result of which the dissociationof AIBN is accelerated and the number of free radicals formed increased.In the critical case the polymerization may “run away”, i.e., becomeuncontrollable. UV polymerization, in contrast, possesses the advantagethat by not irradiating the monomer solution no new free radicals areformed and therefore the polymerization can be controlled more easily.

Depending on the solvent and the photoinitiator used the wavelength tobe irradiated is chosen between 200 and 400 nm. By way of example it ispossible to use standard commercial high-pressure or medium-pressuremercury lamps with an output of, for example, 80 to 200 W/cm.

The free-radical polymerization is advantageously conducted at least upto a conversion of 98% of the monomers.

The polymer is subsequently freed from the solvent in a concentratingextruder. It is preferred here to use a twin-screw extruder operatedeither counterrotatingly or corotatingly. The volatile fractionremaining in the polymer should amount to less than 0.5 weight fraction.The polymer is preferably concentrated under low shear. Through theprocess of the invention it is also possible for the concentratingextruder to be operated at high temperatures without formation of freeradicals as a result of thermal initiators dissociating.

To improve the adhesive properties it is preferred to admix resins tothe acrylate hotmelt. Examples of resins which can be used includeterpene resins, terpene-phenolic resins, C₅- and C₉-hydrocarbon resins,pinene resins, indene resins, and rosins, alone and also in combinationwith one another. In principle, however, it is possible to use anyresins which are soluble in the corresponding polyacrylate; referencemay be made in particular to all aliphatic, aromatic, and alkylaromatichydrocarbon resins, hydrocarbon resins based on single monomers,hydrogenated hydrocarbon resins, functional hydrocarbon resins, andnatural resins.

It is additionally possible to add various extenders (for example,carbon black, TiO₂, solid or hollow beads of glass or other materials),nucleating agents, compounding agents, ageing inhibitors, lightstabilizers, ozone protectants, fatty acids, plasticizers, nucleatingagents, blowing agents, accelerators and/or fillers.

For some applications as a pressure sensitive adhesive it can benecessary to crosslink the polymer, particularly for the purpose ofincreasing the cohesion. For the inventive process it is therefore veryadvantageous to add crosslinkers to the monomer mixture, to the reactionmixture or to the polyacrylate.

As crosslinkers it is possible to use any difunctional or polyfunctionalcompounds known to the skilled worker, whose functional groups are ableto undergo linking reactions with the polyacrylates, particularlyaddition polymerization reactions, polycondensation reactions orpolyaddition reactions. Use is made in particular of difunctional orpolyfunctional acrylates and/or methacrylates, difunctional orpolyfunctional isocyanates or difunctional or polyfunctional epoxides.For UV or EB curing, polyfunctional acrylates are preferred. Thecrosslinkers are preferably metered into the acrylic hotmelt in themelt.

It is also possible to meter in, or to admix to the acrylate melt,substances which crosslink under UV radiation, such as UVphotoinitiators, for example. As photoinitiators it is possible to usebenzophenone, acetophenone, benzil, benzoin, hydroxyalkylphenone, phenylcyclohexyl ketone, anthraquinone, thioxanthone, triazine, or fluorenonederivatives, this list not being conclusive. It is preferred to use typeII photoinitiators. It is also possible, furthermore, to admix anypromoters known to the skilled worker to the acrylic hotmelt which mightmake UV crosslinking more efficient.

The pressure sensitive adhesive prepared by the process of the inventionis suitable with particular advantage for the preparation of, forexample, adhesive tapes. For this purpose the acrylic hotmelt is appliedfrom the melt to a backing in the form of the substance as it is or in aform modified as already described.

As backing materials, for adhesive tapes, for example, it is possible inthis case to use the materials customary and familiar to the skilledworker, such as films (polyesters, PET, PE, PP, BOPP, PVC), nonwovens,foams, woven fabrics and woven films, and also release paper (glassine,HDPE, LDPE). This list is not conclusive.

Following application, the crosslinking can then be carried out in theinventive sense, preferably directly on the backing material, preferablyby means of UV radiation or by means of ionizing radiation, such aselectron beams, for example. In special circumstances it is alsopossible to carry out thermal crosslinking as well.

For UV crosslinking, irradiation takes place in a wavelength range from200 to 400 nm using standard commercial high-pressure or medium-pressuremercury lamps having an output of, for example, 80 to 200 W/cm. For UVcrosslinking it may be appropriate to adapt the lamp output to the webspeed or to carry out partial shading of the web in slow travel in orderto reduce the thermal load thereon. The irradiation time is guided bythe construction and output of the respective lamps. Moreover, it ispreferred to carry out irradiation under an inert gas atmosphere.

The content of the invention further includes the use of the resultantpressure sensitive adhesive for an adhesive tape, it being possible forthe adhesive tape to be provided with a self-adhesive layer on one orboth sides.

EXAMPLES

The invention is illustrated below by examples, without wishing tosubject it to any unnecessary restriction as a result.

A selection is made of acrylic monomers and vinyl monomers depending onthe desired adhesive properties of the acrylic hotmelts.

Test Methods

The following test methods were employed to evaluate the adhesiveproperties of the PSAs prepared.

Shear Strength (Test A)

A strip 13 mm wide of the adhesive tape was applied to a smooth steelsurface which had been cleaned three times with acetone and once withisopropanol. The area of application measured 20 mm×13 mm(length×width). The adhesive tape was then pressed onto the steelbacking four times with a weight of 2 kg. At room temperature a weightof 1 kg was fastened to the adhesive tape, and the time taken for theweight to fall down was measured.

The shear stability times measured are indicated in minutes andcorrespond to the average of three measurements.

180° Bond Strength Test (Test B)

A strip 20 mm wide of an acrylic PSA applied to a polyester was appliedto steel plates which had been washed beforehand twice with acetone andonce with isopropanol. For the measurements on the PE substrate only newplates were used.

The PSA strip was pressed down twice onto the substrate using a 2 kgweight. The adhesive tape was then removed immediately from thesubstrate at a speed of 300 mm/min and at an angle of 180°. Allmeasurements were conducted at room temperature under climatizedconditions.

The results are reported in N/cm and are averaged from threemeasurements.

Determination of the Gel Fraction (Test C)

The carefully dried solvent-free adhesive samples are welded into apouch of polyethylene nonwoven (Tyvek web). From the difference in thesample weights before and after extraction with toluene the gel value isdetermined, i.e., the weight fraction of the polymer that is not solublein toluene.

Samples Investigated

Commercially available substances used Substance Manufacturer Chemicalcomposition Irgacure 819 Ciba Specialty Chemicals Bis(2,4,6-trimethylbenzoyl)phenyl- phosphine oxide Irgacure 651 Ciba SpecialtyChemicals Benzyl dimethyl ketal Vazo 67 DuPont 2,2′-azobis(2-ethylpropiononitrile) Perkadox 16 Akzo NobelBis(4-tert-butylcyclohexyl) peroxydicarbonate Genomer 4212 RahnAliphatic polyurethane acrylate with a functionality of 2 PETIA UCBPentaerythritol triacrylate SR 610 Sartomer Polyethylene glycoldiacrylate with a degree of polymerization of 14 Norsolene M1080 CrayValley Partially hydrogenated HC resin DT 110 DRT Terpene-phenolic resin

Example 1

A mixture of 40 g of acrylic acid, 360 g of 2-ethylhexyl acrylate and400 g of acetone was blended with 2 g of Irgacure 819™ (Ciba Geigy) andrendered inert with nitrogen gas for 45 minutes in a conventional 2 Ireactor with a propeller stirrer. The polymerization was initiated usinga Bluepoint™ UV radiation source (340 nm wavelength, Spectrum).Irradiation was carried out from above into the monomer solution. Theirradiation sequence was chosen in accordance with the followingprogram: Time Duration Irradiation sequence Start to 20 min 19.5 min Ineach case 30 s irradiation, 60 s without irradiation up to 1 h   40 minIn each case 30 s irradiation, 30 s without irradiation up to 2 h   60min In each case 60 s irradiation, 30 s without irradiation

The polymer was freed from the solvent in a drying cabinet under reducedpressure at a temperature of 50° C. Thereafter the polymer was coated ata rate of 50 g/m² onto a Saran-primed PET film 23 μm thick through aslot die heated to 170° C. The PSA had a bubbly appearance. The adhesivetape was subsequently crosslinked with electron beams (dose 15 kGy) withan acceleration voltage of 230 kV using an electron beam unit fromCrosslinking.

The adhesive properties were analyzed by conducting test methods A andB.

Reference Example 2

A 2 I glass reactor conventional for free-radical polymerizations wascharged with 40 g of acrylic acid, 360 g of 2-ethylhexyl acrylate and300 g of acetone/isopropanol (97:3). After nitrogen gas had been passedthrough the reactor for 45 minutes the reactor was heated to 58° C.,with stirring using an anchor stirrer, and 0.2 g of Vazo 67™ (DuPont)was added. The external heating bath was then heated to 75° C. and thereaction was carried out constantly at this external temperature. Afterreaction times of 1 h and 1.5 h a further 0.2 g of Vazo 67™ was added.After 3 h and 6 h dilution was carried out with in each case 150 g ofacetone/isopropanol mixture (97:3). After 16 h the residual initiatorswere reduced by dropwise addition of 0.4 g of Perkadox 16™ (Akzo Nobel)as a solution in 10 g of acetone. The reaction was terminated after atime of 22 h and the product cooled to room temperature.

The polymer was freed from the solvent in a drying cabinet under reducedpressure at a temperature of 50° C. Thereafter the polymer was coated ata rate of 50 g/m² onto a Saran-primed PET film 23 μm thick through aslot die heated to 170° C. The PSA had a smooth appearance. The adhesivetape was subsequently crosslinked with electron beams (dose 15 kGy) withan acceleration voltage of 230 kV using an electron beam unit fromCrosslinking.

The adhesive properties were analyzed by conducting test methods A andB.

Example 3

A mixture of 4 g of acrylic acid, 32 g of N-tert-butylacrylamide, 4 g ofmaleic anhydride, 180 g of 2-ethylhexyl acrylate, 180 g of n-butylacrylate and 400 g of acetone was blended with 2 g of Irgacure 819™(Ciba Geigy) and rendered inert with nitrogen gas for 45 minutes in aconventional 2 I reactor having a propeller stirrer. The polymerizationwas initiated using a Bluepoin™ UV radiation source (340 nm wavelength,Spectrum).

Radiation was carried out from above into the monomer solution,proceeding in analogy to the irradiation program in Example 1.

The polymer was blended in acetone with 30 weight fractions of DT 110™(DRT) and 1 weight fraction of SR 610 (Sartomer) and was freed from thesolvent in a drying cabinet under reduced pressure at a temperature of50° C. Thereafter the polymer was coated at a rate of 100 g/m² to aSaran-primed PET film 23 μm thick via a slot die heated to 170° C. Theadhesive tape was subsequently crosslinked with electron beams (dose 50kGy) at an acceleration voltage of 230 kV using an electron beam unitfrom Crosslinking. The adhesive properties were analyzed by conductingtest methods A and B. In parallel thereto a specimen was stored in thedrying cabinet at 140° C. for 8 hours and then test method C was carriedout.

Reference Example 4

A 2 I glass reactor conventional for free-radical polymerizations wascharged with 40 g of acrylic acid, 360 g of 2-ethylhexyl acrylate and300 g of acetone/isopropanol (97:3). After nitrogen gas had been passedthrough the reactor for 45 minutes the reactor was heated to 58° C.,with stirring using an anchor stirrer, and 0.2 g of Vazo 6₇™ (DuPont)was added. The external heating bath was then heated to 75° C. and thereaction was carried out constantly at this external temperature. Afterreaction times of 1 h and 1.5 h a further 0.2 g of Vazo 67™ was added.After 3 h and 6 h dilution was carried out with in each case 150 g ofacetone/isopropanol mixture (97:3). After 16 h the residual initiatorswere reduced by dropwise addition of 0.4 g of Perkadox 16™ (Akzo Nobel)as a solution in 10 g of acetone. The reaction was terminated after atime of 22 h and the product cooled to room temperature.

The polymer was blended in acetone with 30 weight fractions of DT 110™(DRT) and 1 weight fraction of SR 610 (Sartomer) and was freed from thesolvent in a drying cabinet under reduced pressure at a temperature of50° C. Thereafter the polymer was coated at a rate of 100 g/m² onto aSaran-primed PET film 23 μm thick through a slot die heated to 170° C.The adhesive tape was subsequently crosslinked with electron beams (dose15 kGy) with an acceleration voltage of 230 kV using an electron beamunit from Crosslinking.

The adhesive properties were analyzed by conducting test methods A andB. In parallel thereto a specimen was stored in the drying cabinet at140° C. for 8 hours and then test method C was carried out.

Example 5

The procedure of Example 3 was repeated. The polymer prepared by UVpolymerization was blended with 30 weight fractions of Norsolene M1080™(Cray Valley) and 1 weight fraction of Genomer 4212™ (Rahn).

Reference Example 6

The procedure of Example 4 was repeated. The polymer prepared by UVpolymerization was blended with 30 weight fractions of Norsolene M1080™(Cray Valley) and 1 weight fraction of Genomer 4212™ (Rahn).

Example 7

The procedure of Example 3 was repeated. UV polymerization was carriedout using 2% of acrylic acid, 49% of 2-ethylhexyl acrylate and 49% ofn-butyl acrylate. The polymer was blended with 30 weight fractions ofNorsolene M1080™ (Cray Valley) and 1 weight fraction of PETIA™ (UCB).

Reference Example

The procedure of Example 4 was repeated. UV polymerization was carriedout using 2% of acrylic acid, 49% of 2-ethylhexyl acrylate and 49% ofn-butyl acrylate. The polymer was blended with 30 weight fractions ofNorsolene M1080™ (Cray Valley) and 1 weight fraction of PETIA™ (UCB).

Results

Comparison of Examples 1 and 2 illustrates that residual initiatorswhich remain in the polymer, such as Vazo 67™ from Du Pont, for example,produced constituents in the PSA which dissociate at high temperaturesin the course of the coating operation and so give rise to nitrogenbubble inclusions. Through UV-initiated polymerization this can be ruledout (see Example 1).

As a result of the UV-initiated polymerization, moreover, a differentmolecular weight and also a different molecular weight distribution areachieved. This is reflected in the different adhesive properties.Moreover, in Example 2, the adhesive properties are also adverselyaffected by the bubbly appearance (see Table 1). TABLE 1 BS steel SST,10 N EB dose Example [N/cm] [min] [kGy] 1 3.8 >10,000 15 2 3.9 6,795 1550 g/m² application rateSST: Shear stability time at RTBS: Bond strength

Example 1 exhibits a much higher shear strength for the samecrosslinking dose. In Examples 3 to 8 the gelling behavior due to theresidual initiators was investigated. Investigations were carried outexclusively on polymers blended with resin. Additionally, differentpolyfunctional acrylates were added in order to ensure electron beamcrosslinkability.

For each example, as a reference, a polymer prepared conventionally by athermally free-radical polymerization was executed. All examples werefirst subjected to thermal storage, and then the gel value was measured.The results are listed in Table 2. TABLE 2 Gel value Example [%] 3 0 4 55 0 6 8 7 0 8 14

Table 2 shows that for all of the UV-initiated polymerizations,independently of the resin used or of the crosslinker, no gel fractioncould be detected. The reference specimens prepared in comparisonthereto, in contrast, had a gel fraction of 5 to 14%—a fraction whichdoes not allow clean coating from the melt in the hotmelt operation.

Furthermore, the adhesive properties of the specimens prepared werecompared with one another after the coating operation in Table 3. TABLE3 BS steel SST, 10 N EB dose Example [N/cm] [min] [kGy] 3 11.4 5,145 504 12.1 3,320 50 5 9.4 4,765 50 6 10.1 2,920 50 7 10.2 3,005 40 8 10.61,855 40100 g/m² application rateSST: Shear stability time at RTBS: Bond strength

There is hardly any variation in the adhesive properties of the examplesdepicted. The specimens prepared by UV-initiated polymerization possessa somewhat higher shear strength. On the other hand, the bond strengthis situated on average at an only slightly lower level.

1. A process for preparing pressure sensitive adhesives based onacrylate hotmelt, in which a monomer mixture including at least thefollowing monomers (a) 70 to 100% by weight of compounds selected fromthe group consisting of (meth)acrylic acid and the derivatives thereofcorresponding to the following formula

 with R₁═H or CH₃ and R₂=an alkyl chain having 2 to 20 carbon atoms andalso including (b) 0 to 30% by weight of olefinically unsaturatedmonomers containing functional groups and also including (c) if desired,optionally, further components is free-radically polymerized in solutionto give a polyacrylate, wherein I. the polymerization is initiated usingat least one dissociating photoinitiator and by irradiation withultraviolet light, the photoinitiator being added to the monomer mixturebefore the beginning of the polymerization and/or to the reactionmixture in the course of the polymerization, II. the polyacrylate isfreed from the solvent, III. the polyacrylate is processed further inthe melt.
 2. The process of claim 1, wherein the at least onephotoinitiator is or are used in an amount of from 0.1 to 2% by weight,based on the weight of the monomer mixture.
 3. The process of claim 1,wherein the irradiation with ultraviolet light is carried out in theform of two or more irradiation cycles and/or pulsed irradiation iscarried out.
 4. The process of claim 1, wherein the free-radicalpolymerization is conducted at least up to a conversion of 98% of themonomers.
 5. The process of claim 1, wherein the removal of the solventis carried out after the polymerization in a twin-screw extruder.
 6. Theprocess of claim 1, wherein crosslinkers are added to the monomermixture, to the reaction mixture or to the polyacrylate.
 7. The processof claim 1, wherein resins and/or additives are added to the monomermixture, to the reaction mixture or to the polyacrylate.
 8. The processof claim 1, further comprising the step of applying the pressuresensitive adhesive from the melt to a backing material.
 9. An adhesivetape provided with a layer of a self-adhesive prepared by the process ofclaim 1 on one or both sides.
 10. The process of claim 2, wherein saidat least one photoinitiator is used in an amount of between 0.25 and 1%by weight, based on the weight of the monomer mixture.
 11. The processof claim 6, wherein said crosslinkers are selected from the groupconsisting of difunctional or polyfunctional acrylates, difunctional orpolyfunctional methacrylates, difunctional or polyfunctionalisocyanates, difunctional or polyfunctional epoxides, or mixturesthereof.
 12. The process of claim 7, wherein said resins and/oradhesives are selected from the group consisting of ageing inhibitors,light stabilizers, ozone protectants, fatty acids, plasticizers,nucleating agents, blowing agents, accelerators and fillers.